This invention relates generally to high-speed laser electrographic printers, and more particularly to a method and apparatus for locking the transparent cylinder used as a forms overlay drum in such printers to the drive hubs upon which it is mounted.
Forms overlays are particularly suited for printing tasks where variable data or images are to be added to fixed data or images. For example, the thousands of bank statements provided to depositors are fixed images to which the variable debit and credit data of the many individual depositors is added or integrated. However, this bank statement printing chore is comparatively straight forward because the forms contains blank or white spacers for receiving the variable debit and credit data. That is, the variable data doesn't overlie the fixed data. A more complex task is overprinting a traveler's route onto a complex map where two images or sets of data are printed in the same area on the paper or other support member.
The forms overlay technique is advantageous to electronic printing and computer printing in particular because its use saves the electronic memory storage space required to electronically print the form for other electronic control functions. Often this saving in memory space yields a meaningful cost improvement to the entire printing system.
Conventional laser electrographic printers utilize a negative-on-negative approach for forms overlay. The Xerox Corporation Xerox 1200 computer printer, the Siemens Corporation Siemens 3352 laser printer, and the International Business Machines Corporation IBM 3800 printing subsystem are examples of negative-on-negative or negative overlay electrographic printers. Briefly, in each, a latent electrostatic image is generated in response to electronic signals on a charged photoconductive surface by exposing the charged surface to electromagnetic radiation, such as light from a laser, thereby discharging the surface in areas struck by the light. The discharged areas against the undischarged background define the negative latent electrostatic image, while the inverse defines a positive latent electrostatic image. The discharged areas are analogous to white markings on a black background for a reflection image or transparent markings on an opaque background for a transparency image. The negative latent image is made visible or developed by depositing black toner onto the discharged areas and transferring the black toner to white paper. The development and transfer steps convert the negative latent electrostatic image to a positive reflection image. The toner image printed from the electronic signals is hereinafter referred to as an electronic image or print. Further information pertaining to the IBM 3800 printing subsystems and related support systems is available in the IBM publications GA32-0049-0, "Introducing the IBM 3800 Printing Subsystem Model 3, " GA32-0050- 1, "Reference Manual for the IBM 3800 Printing Model 3," and GH20-9158-4, "Document Composition General Information," which are incorporated herein by reverence.
The "forms overlay" printer or image is created by the above commercial computer printers by exposing the charged photoconductive surface to a light image of a negative reflection print or negative transparency print or overlay. When the negative overlay is flooded with light and the reflected or transmitted light strikes the photoconductor, a negative latent electrostatic image is created. The overlay latent image is composed of discharged areas representing markings on a charged background. The development and transfer of the overlay latent image inverts or converts the negative sense of the image to the positive sense as in the case of the above described electronic image.
The overlay and electronic images are generated during a single cycle of the electrographic computer printers. Each of these printers employ a rotating photoconductive drum and electronic overlay images are generated during a single cycle or rotation of the drum prior to the development step. This means that the image is superimposed over the other on the same area of the photoconductive drum. This superimposition is possible because of the negative mode of operation. The first image (e.g., the electronic image) discharges the drum only in information areas and not background areas. Statistically, the information areas occupy significantly less space than the background areas. Consequently, the second image in this example the overlay image, is recorded in the charged background regions of the electronic image. In other words, the second image can be produced only if the first left significant areas of charge on the drum surface on which the second image can be recorded.
In other laser electrographic printers, such as the 6100 Printing Subsystem of Storage Technology Corporation, the negative-on-negative approach to forms overlay is accomplished in a somewhat different manner. A film negative of the form to be printed is mounted on a transparent cylinder, known as the forms overlay drum, which is rotated synchronously with the photoconductive drum at the same circumferential speed. The image of the form negative is projected onto the photoconductive drum by means of a fluorescent lamp inside the forms overlay drum and a fixed optical system. In this way, a copy of the image on the negative is printed or superimposed on the data printed by the laser printing system. Further information pertaining to the 6100 Printing Subsystem and related support systems is available in the Storage Technology Corporation publications P/N 3825131501, "6100 Printing Subsystem Paper Specification", P/N 3825131601, "6100 Printing Subsystem Forms Design Guide", P/N 3825118701, "6100 Printing Subsystem Operators Manual and Product Description Manual", and EP-064-0, "6100 Printing Subsystem Consumables and Accessories Catalog", which are incorporated herein by reference.
One particular problem encountered with printers of the type utilizing form overlay drums, such as the 6100 Printing Subsystem of Storage Technology Corporation, is that the transparent cylinder of such drums often fail due to stress cracking at the screw mounting holes on the drive hubs. Initially, the transparent cylinder was attached to the drive hubs through a pinned arrangement. Such an arrangement, however, did not sufficiently lock the transparent cylinder to the drive hubs which resulted in cylinder slippage causing misregistration of the image from the film negative with respect to the electronic image superimposed thereon. As is readily apparent to one of ordinary skill in the art, such slippage would lead to a torque loading between the cylinder and hubs due to the torsion which would be experienced by the slipping, thin-walled cylinder. It was, therefore, desirable to provide an improved method and apparatus for locking the transparent cylinder to its respective drive hubs.
Subsequent efforts to provide an improved forms overlay drum resulted in cementing of the transparent cylinder to its drive hubs. After the cement had dried, the transparent cylinder, typically comprised of Plexiglas.RTM. (a registered trademark of the Rohm and Haas Company), was mechanically locked to the drive hubs with a pair of flat head screws countersunk in the drum. Such an arrangement, however, in operation proved to be deficient in spite of its improved locking nature. Due to the extreme thermal changes brought on by the fluorescent lamp inside the forms overlay drum, the cement used to bind the transparent cylinder to its respective drive hubs quickly becomes brittle and breaks loose, thereby putting more stress on the flat head screws used to mechanically lock the cylinder to the hubs. Thereafter, shock loading and vibration experienced during operation of the printer causes undue stress on the joints which lock the two parts together, often resulting in stress cracks in the transparent cylinder at the countersunk holes. It would, therefore, be desirable to provide an improved method and apparatus for locking the transparent cylinder to its respective drive hubs which is capable of withstanding thermal changes and shock loading without putting undue stresses on the joint.